speaker box question

For a sealed box, you'd want something has 3.375 cu. ft. of internal volume; for a ported box, you'd want one with 5.25 cu. ft. of internal volume, and 3 ports that are 4" in diameter and 16.2" long.  According to JL, a triple sub enclosure just requires 3 times the amount of internal volume and 3 times as many ports, so that's how I came up with the above figures.

They don't really mention which type of enclosure will sound "better", but then again that's always a pretty subjective thing anyways.  They do say this at the very beginning of the manual though: 

Both types of enclosures will give you excellent sound quality and

low-frequency extension. The sealed enclosure is a better choice if space is extremely tight, but the ported

enclosure will produce 3dB more output at the same power if you have the extra space to work with.

It's really hard to give a definite answer cuz as you know there's so many variables and what sounds good in one vehicle can sound like poo in another vehicle.  I think a ported box built to JL's recommended specifications would not suffer much of a SQ hit at all, or else they would have noted that in the manual.  It might go a little deeper too, but once again that's hard to say for sure.

I don't know how much (if any) of a difference a slightly larger sealed box would sound compared to your 3 cu. ft. box.  If it'll cost you $300 to get a new box then I'd say just stuff some poly-fill in your existing box and it'll act like it's pretty darn close to a 3.375 cu. ft. box; and it'll cost a whole lot less obviously.